A number of operating procedures for the production of hollow bodies from thermoplastic material by an extrusion blow molding process, particularly in the case of producing relatively large hollow bodies such as barrels or motor vehicle tanks, involve the use of at least two blow molding molds associated with an extrusion head. The blow molding molds are moved selectively into a receiving position, for example beneath an extrusion head, in order to receive preforms which are extruded from the extrusion head. In that operation it is necessary for the blow molding molds to be arranged movably relative to the extrusion head, in such a way that, after a preform has been received by a blow molding mold, that mold is moved away in order to liberate the space beneath the extrusion head for the other blow molding mold to receive the respective following preform.
In continuous extrusion of the preforms, it is generally usual for the movement of the blow molding mold between the receiving station beneath the extrusion head and a station which is spaced away from the receiving station to take place substantially parallel to the separation surface of the blow molding mold. When the blow molding mold parts are of a symmetrical configuration, that separation surface is the vertical plane of symmetry of the mold, being at any event a surface which extends not parallel but mostly perpendicularly to the directions of the mold opening and closing movements. The need for the movements of the blow molding mold between the receiving position and a position spaced therefrom to take place approximately parallel to the above-mentioned plane makes it possible for the two parts of the blow molding mold, in the opened condition thereof, upon movement into the receiving position, to be guided laterally past the preform which is in the process of being formed and which may already involve the major part of its required length. In that respect, it is also possible for the blow molding mold to move along an arc portion as long as the radius of that arc is of such a size that the above-indicated requirements are met. Nonetheless, in a continuous extrusion procedure, the possible movements as between the blow molding mold and the preform which is in the receiving station are limited, unless particular additional steps are taken, for example by introducing the preform into the blow molding mold in the receiving station, by means of a gripper device for firstly removing the preform from the extrusion head which is disposed at a certain spacing from the receiving station. Thus, when such a mode of operation is employed, the extrusion head could be arranged at a spacing above the blow molding mold which is in the receiving station, that spacing approximately correspondingly to the target or reference length of the preform.
The limitations in regard to performance of the movements of the blow molding molds, which are imposed in continuous extrusion of the preform, can have a disadvantageous effect insofar as the operation of removing the hollow bodies which are produced by being expanded in the blow molding molds cannot be effected in a position which is common to the two blow molding molds and which is selectively adopted by the respective blow molding molds. On the contrary, in the above-indicated apparatuses the usual practice is that the blow molding molds, upon removal of the hollow bodies therefrom, adopt different positions, with the consequence that the devices required for removal of the hollow bodies and for further transportation thereof into some downstream-disposed treatment stations must be duplicated. That gives rise to a not inconsiderable level of expenditure, which increases with the size of the hollow bodies, as the transportation distances, removal devices, and downstream-disposed transportation means and other devices must inevitably be adapted to the size of the hollow bodies.
Moreover, when dealing with hollow bodies of relatively large size, for example motor vehicle tanks, it, is frequently necessary to introduce additional parts into the blow molding mold prior to closure thereof, for receiving the preform; such insertion parts when introduced into the mold are joined to the hollow body in the mold in the course of the operation of expanding the preform to constitute the hollow body. The insertion parts for example may involve installation components which are to be mounted in the interior of a tank, but they may also be reinforcing parts which are to be mounted on the outside. At any event the use of such insertion parts which must be introduced into the mold prior to closure thereof is of great practical significance, insofar as those parts must be correctly positioned in the blow molding mold with a great deal of care and generally in a very short time. The endeavor here is also that the at least two blow molding molds use a common station in which they are provided with the insertion parts to be introduced into the molds, so that essential members of the devices which are required for that operation can be jointly used by at least two blow molding units.
In discontinuous extrusion using an accumulator or storage head, the limitations and disadvantages described hereinbefore in relation to continuous extrusion are less pronounced, as there is greater freedom in regard to the movements of the at least two blow molding units. It is also possible for the two blow molding units to have a common station in which the hollow bodies are removed from the respective blow molding unit disposed at that station. A similar consideration applies in regard to the operation of introducing any parts which are to be joined to the hollow body in the blow molding mold. However discontinuous extrusion also requires the fulfilment of certain conditions, if for example the arrangement provides for both blow molding units a single common position in which the operation of removing the hollow bodies is effected.